**Core Concept**
Graves' disease is an autoimmune disorder characterized by the production of autoantibodies that stimulate the thyroid-stimulating hormone receptor (TSH-R), leading to hyperthyroidism. This autoimmune response is mediated by helper T cells (Th cells) and B cells, resulting in the production of thyroid-stimulating immunoglobulins (TSI).

**Why the Correct Answer is Right**
Graves' disease is an example of a type II hypersensitivity reaction, also known as cytotoxic hypersensitivity. In this reaction, autoantibodies bind to specific antigens on the surface of thyroid cells, leading to the activation of the thyroid gland and increased production of thyroid hormones. The binding of autoantibodies to the TSH-R triggers a series of intracellular signaling pathways that ultimately result in the increased production of thyroid hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because type I hypersensitivity reactions are mediated by IgE antibodies and are typically associated with allergic reactions.
**Option B:** This is incorrect because type III hypersensitivity reactions involve the formation of immune complexes and are typically associated with conditions such as systemic lupus erythematosus.
**Option C:** This is incorrect because type IV hypersensitivity reactions are cell-mediated and are typically associated with conditions such as contact dermatitis.

**Clinical Pearl / High-Yield Fact**
Graves' disease is the most common cause of hyperthyroidism in iodine-sufficient areas, and it is often associated with other autoimmune disorders such as Hashimoto's thyroiditis and type 1 diabetes.

**Correct Answer: C. Type II hypersensitivity reaction. Type II hypersensitivity reaction.**